Spectrographs including plane diffraction grating for dispersing light into wavelength-components have been proposed. Such spectrographs usually include two concave mirrors, of which one is used to collimate the light passing through the entrance slot of the spectrograph, and the other is used to focus the light dispersed by the plane grating as a spectrum. Spherical, concave mirrors are mostly used, as they are easily manufactured with the necessary exactness at the optical surface.
The use of concave mirrors, however, means that the impinging light will form an angle with respect to a normal of the mirror surface, which will introduce aberrations in the spectrum. The two most noticeable aberrations are called astigmatism and coma. The firstmentioned one means that a pointshaped entrance opening will show up as a line upon the focal surface, and the other one, that the point will be located assymetrically and having a cometshaped shading. In a spectrograph, where a spectrum is obtained in one dimension upon a photographic plate, astigmatism will not be particularly detrimental, as it will only mean an elongation of the image of the slot in the focal plane. Coma, on the other hand, means a reduced resolving power, as the image of the slot will have a comatic shading to one of its sides, resulting in an assymetric profile of the spectral line. M. Czerny & A. F. Turner (Z. Phys. 61, 792 (1930)) have shown that the comatic aberration can be relieved by spacing the mirrors, and locating them in such angular positions, that the aberration caused by one mirror will be counteracted by the other mirror. Such arrangement is nowadays used in most plane grating spectrographs having spherical concave mirrors.
When the plane grating is of the Echelle type (Spectrochimica Acta 6, 61, (1953)), which will produce spectra of high spectral orders, and is combined with a further wavelength-dispersing, optical member, mounted in such a manner that its wavelength dispersion is perpendicular to that of the Echelle grating, the resulting spectrum will be divided into segments arranged below each other in the focal plane. In such Echelle-spectrographs it is important, that the image is free of astigmatism, as otherwise the spectral segments would overlap each other.
That may be further accentuated in an instrument, where the focal surface is the photo cathode of an image tube, as the size of the photo cathode is limited. Images compensated with respect to astigmatism and coma have been obtained by means of expensive un-spherical mirrors, which are difficult to manufacture, or by a noticeable reduction of the height of the grating, which results in a reduced luminous power.
By means of the present invention it is possible to obtain a stigmatically and comatically compensated spectrum in a plane grating arrangement including spherical mirrors without any reduction of the height of the grating being necessary.